1. Field of the Invention
The present invention relates to a jitter detection apparatus and a jitter detection method used for reproducing information recorded on an optical disc or the like.
2. Description of the Related Art
As digital signal processing technologies progress, binary information recorded on an information recording medium such as, for example, an optical disc is reproduced using a binarization digital read channel technology. According to the binarization digital read channel technology, binarization is performed after a reproduction waveform is converted into digital data. Since the digital read channel technology uses digital data which is sampled on a time series, jitter, which is an index representing the signal quality, cannot be directly obtained. When the digital read channel technology is used, jitter detection methods described in, for example, Japanese Laid-Open Publication Nos. 2002-107394 and 2002-15523 are used. By the method described in Japanese Laid-Open Publication No. 2002-107394 (paragraph 0016 and FIG. 6), the jitter is obtained by normalizing the digital data sampled at a zero-cross point using digital data immediately before or immediately after the zero-cross point. By the method described in Japanese Laid-Open Publication No. 2002-15523 (paragraphs 0017 to 0020 and FIG. 11), the jitter is obtained by performing linear interpolation using digital data before and after the zero-cross point.
Recently, in order to further increase the recording capacity of information recording mediums, the adaptation of a new modulation system is considered instead of the conventional modulation system. Whereas the conventional modulation system sets the minimum run-length as 3 clock cycles (for example, an EFM modulation system used for CDs or an 8–16 modulation system used for DVDs), the new modulation system sets the minimum run-length as 2 clock cycles. As the run-length is shorter, the recording density is increased, but the influence of inter-symbol interference is also increased. Due to such an influence of inter-symbol interference, it has become impossible to obtain correct jitter using the conventional jitter detection method when data sampled at a sampling frequency at or close to channel rate is reproduced. The reason is that the influence of inter-symbol interference is too large for linear interpolation to be used for calculating the jitter.